1. Field of the Invention
The present invention relates to ultrasonic bonding machines and more particularly concerns an open center mounting that permits optimum substantially linear motion of a bonder tip.
2. Description of Related Art
In the manufacture of microelectronic circuit packages, certain electrical connections are made with extremely thin gold or aluminum wires. For example, connections between a semiconductor integrated circuit chip and package leads are typically made with such thin wires. As another example, active elements in semiconductor hybrid circuits may be interconnected with extremely thin wire.
Attachment of the interconnecting thin wires is commonly accomplished with an ultrasonic wire bonding machine which supplies wire and makes connections of the wire leads. Such a machine generally includes a wire bonding head which can move vertically, horizontally and rotatably about a vertical axis. An ultrasonic transducer, carrying a bonding tool, is pivotally mounted to the bonding head so that as the tool comes into contact with the workpiece it may be pivotally displaced through a very short distance about a horizontal axis, as may be required to ensure proper contact and pre-load. The transducer includes an arm to which the bonding tool is attached. Linear vertical movement of the bonding head raises or lowers the tool to a position in which the tool tip is close to but spaced from the workpiece. As the tool is finally lowered or initially raised to cause the bonding tip to contact or withdraw from the workpiece, the tool tip must be able to undergo a small vertical motion relative to the machine frame to accomplish or release the small tool preloading. This motion previously has been provided by pivotal motion of the transducer and its tool tip. However, such a pivotal motion introduces a small amount of horizontal motion, commonly termed "tip skid", in the course of the desired vertical preloading motion of the tip. Such horizontal component of motion causes a corresponding horizontal force component to be exerted on the wire which has been or is to be bonded, tending to improperly and undesirably displace the wire.
These undesired horizontal motion components are increased by the location of the tool tip pivot well above the workpiece, as is required to provide clearances required for the usual occupied center mounting. Therefore, if the pivot axis for the preloading motion of the tool tip is positioned above the plane of the workpiece, the adverse tip skid effect of the pivotal motion is increased.
My prior patent, U.S. Pat. No. 4,598,853, for Open Center Flexural Pivot Wire Bonding Head, describes a flexural pivot structure that employs a pair of leaf springs to provide a flexural mounting of the transducer and tool tip. This spring mount defines a vertical motion with minimized tip skid. The arrangement of my prior patent has been found to be efficient and effective, but greater rigidity and stability of the movable mounting for the ultrasonic transducer will provide improved efficiency, repeatability and reliability.
Accordingly,it is an object of the present invention to provide an open center mounting that provides some or all of the above-mentioned improvements.